In general, a conventional motor driver of driving a motor installed in the compressor of a refrigerating and air conditioning system has the following structure. The motor driver includes a rectifier circuit for rectifying the AC voltage input from the AC power source, and an inverter for driving the motor coupled to this rectifier circuit. A smoothing capacitor with a sufficiently large capacitance is coupled between the output terminals of the rectifier circuit.
This driver detects the rotational position of the motor rotor based on the back electromotive force produced in the motor stator coil while the motor rotates or based on the motor current, without relying on a special position-detecting sensor such as a Hall element and encoder. The driver uses this detection signal for sequentially switching power distribution among the switching elements in the inverter to drive the motor by controlling the commutation of the current traveling in the stator coil.
This type of drive method which does not use a special position-detecting sensor is generally called a sensorless drive. Motors installed in the compressors of refrigerating and air conditioning systems adopt this sensorless drive since it is very difficult to provide a position-detecting sensor in a high-temperature and high-pressure atmosphere, refrigerant atmosphere, or oil atmosphere inside the compressor.
More recently, attempts to drastically reduce the capacitance of the smoothing capacitor in the rectifier circuit have been made to downsize the motor driver, as typically disclosed in Japanese Patent Unexamined Publication No. 2002-51589.
This type of conventional motor driver is described next with reference to a drawing. FIG. 6 is a block diagram of a conventional motor driver. The output of single-phase AC power source 1 is coupled to diode full-wave rectifier circuit 2. The output of this diode full-wave rectifier circuit 2 is coupled to smoothing capacitor 3. This smoothing capacitor 3 has a very small capacitance of about 1% of a conventional capacitor.
PWM (pulse-width modulation) inverter 4 is coupled to both ends of this smoothing capacitor 3. This PWM inverter 4 includes six switching elements (including a reverse diode) in a three-phase bridge connection. Three-phase windings are provided around the stator of motor 5. Each of one end of the three-phase windings is respectively coupled to the output of PWM inverter 4. Accordingly, motor 5 is driven by PWM inverter 4.
As shown in FIG. 6, control circuit 6 receives information including the voltage of single-phase AC power source 1, DC-part current, output current of PWM inverter 4, and positional information on position-detecting sensor 7; and controls the gates of six switching elements configuring PWM inverter 4 so as to optimally drive the motor.
In the above conventional structure, the rotor position is detectable even if the DC voltage applied to the inverter drops when an encoder or Hall element is employed as a position-detecting sensor. However, when it is not feasible to install a position-detecting sensor, such as in the case of a compressor, the above conventional structure is not applicable.
In general, known methods of driving a brushless DC motor without using a special position-detecting sensor (sensorless drive method) include a method of detecting the rotor position based on the back electromotive force produced in the motor stator coil and a method of detecting the rotor position based on the motor current.
In a motor driver that adopts one of the above conventional sensorless drive methods, the rotor position is detectable only when the capacitance of the smoothing capacitor is sufficiently large and the ripple voltage in the voltage across the smoothing capacitor (the output voltage of the rectifier circuit) is small. This is because the back electromotive force or motor current is stable enough to reliably detect the rotor position only when the capacitance of the smoothing capacitor is sufficiently large and the ripple voltage in the output voltage of the rectifier circuit is small.
If the capacitance of the smoothing capacitor in a motor driver adopting the conventional sensorless drive method is drastically reduced so as to downsize the entire unit to the size of the driver using a special detecting sensor as shown in FIG. 6, the ripple voltage drastically increases. This makes the back electromotive force needed for detecting the rotor position undetectable; or the motor current needed for detecting the rotor position not to flow when the voltage applied to the PWM inverter is low.
Consequently, accurate detection of the rotor position becomes difficult, leading to drastic deviation in commutation timing by the PWM inverter. This results in the application of high motor currents as well as reducing the motor's efficiency. In the worst case, a failure due to the motor stopping may occur.